With the recent tendency towards miniaturization of electronic equipment, a strong demand has arisen for miniaturizing electronic parts and there is also an increasing demand for reducing the size of multilayer ceramic capacitors. To meet these needs, dielectric ceramics are required to satisfy the following conditions for electrical characteristics, i.e., high dielectric constant, small dielectric loss and high insulation resistance. In order to miniaturize devices, the thickness of individual layers of a multilayer ceramic capacitor must be reduced but with conventional ceramic compositions for ceramic capacitors, there has been a limit on the efforts to reduce the thickness of each layer since the grains of a sintered body are large enough to form residual pores. A further problem in the efforts toward size reduction and larger capacity is that the inevitable increase in electrode areas leads to a higher cost if conventional Pd-based materials for internal electrodes are used. Several patents have issued on three-component ceramic compositions that contain Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 (hereinafter abbreviated as PMN), Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 (hereinafter abbreviated as PNN) and PbTiO.sub.3 (hereinafter abbreviated as PT) but each of the proposals has necessitated a high sintering temperature. For example, Japanese Patent Publication No. 13962/1981 proposed a PMN-PNN-PT based solid solution that attained a relative dielectric permittivity of 23000-24000 but the sintering temperature was as high as 1230.degree.-1260.degree. C. Japanese Patent Publication No. 28619/1986 shows the characteristics of a two-component (PMN-PT) ceramic composition. This composition has a dielectric constant as high as 20000-21000 but the sintering temperature is also as high as 1100.degree. C. and above and it has been difficult to manufacture a multilayer ceramic capacitor in which an inexpensive Ag/Pd based metal with a large proportion of the Ag component is used as an internal electrode material. Thus, none of the compositions have yet found commercial use on account of limitations from the viewpoint of costs.
The electrical characteristics that are to be possessed by ceramic compositions are high dielectric constant, small dielectric loss and high insulation resistance. However, BaTiO.sub.3 based materials which have so far been used extensively have the problem that if one attempts to increase their dielectric constant, the temperature-dependent change in dielectric constant will increase and that if one attempts to reduce the size of grains in the sinter, the sintering temperature will increase. Thus, it has been difficult to satisfy the various necessary characteristics for the above-mentioned multilayer ceramic capacitor at sintering temperatures of 1100.degree. C. and below where Ag-rich Ag/Pd alloys can be used as internal electrodes.
It is necessary that ceramic materials that have high dielectric constant, low dielectric loss and high insulation resistance and that are characterized by small grains in the sinter and a small number of pores should be developed as dielectric compositions for use in multilayer ceramic capacitors. Such characteristic requirements must be satisfied by ceramics that are sintered at temperatures not higher than 1100.degree. C. and only when this condition is met, the recent need for producing multilayer ceramic capacitors of a smaller size with a larger capacity can be satisfied.
The present invention has as an object providing a dielectric ceramic composition that insures the above-mentioned characteristics to be exhibited in a desired manner even if it is sintered at temperatures not higher than 1100.degree. C.